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Reinsurance Reinsurance (Effect Of Reinsurance And Retrocessional Reinsurance On Losses) (Details) - Underwriting [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Effects of Reinsurance [Line Items]
Gross	$ 868,069	$ 893,276	$ 2,111,626	$ 1,715,030
Ceded	(154,307)	(215,389)	(321,487)	(349,412)
Net losses and loss adjustment expenses	$ 713,762	$ 677,887	$ 1,790,139	$ 1,365,618

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Amount, before effect of policies assumed or ceded, of expense related to provision for policy benefits and costs incurred.

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Amount, after effects of policies assumed or ceded, of expense related to the provision for policy benefits and costs incurred.

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Amount of reduction of provision for policy benefits and costs incurred for policies ceded.

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